1. Field of the Invention
The present invention relates to a method of driving and controlling an ink jet print head, an ink jet print head applied to this method, and a printing apparatus using this method. Specifically, the present invention relates to an ink jet print head and printing apparatus based on a thermal ink jet system in which heating resistance elements that generate heat in response to electric conduction are used to cause film boiling in ink so that growth and contraction of the resulting bubbles is used to eject the ink through nozzles.
2. Description of the Related Art
In recent years, ink jet printing systems have advanced rapidly because they can achieve high-density and high-definition printing at a high speed to promptly provide high-quality print matter and are suitable for multicolor applications and size reductions.
An example of a head for use in such an ink jet printing system is what is called a side shooter type ink jet print head constructed so that ink passages are bent toward the nozzles and a thermal operation portion is arranged in each bent portion and opposite the corresponding nozzle.
FIG. 1 shows an example of the construction of a side shooter type print head. This head has a plurality of nozzles 3001 arranged in zigzag configuration at the opposite sides of an ink supply port 3003 opened in a substrate 3005 composed of silicon, and heating resistance elements 3002 arranged on the substrate 3005 for each of the ink passages 3004 and used to eject ink droplets in order to generate thermal energy.
The heating resistance elements 3002 are each composed of a heater consisting mainly of HfB, TaN, TaAl, TaSiN, or the like, electrode wiring consisting of Al, AlCu, AlSi, or the like to supply power to the heater, and a protective film consisting of SiC, SiO, SiN, Ta, or the like to protect the heater and electrode wiring from ink (not shown).
The ink supply port 3003 is generally formed using a dicing process, a sand blast process, an anisotropic etching process, or the like. FIG. 1 shows the ink supply port 3003 formed by the anisotropic etching process, which has a high machining precision. The machining precision for the ink supply port 3003 is an important factor for manufacture of print heads. With a low machining precision, the ink passages 3004 have different distances from their ends located closer to the ink supply ports 3003 to the heaters, resulting in a variation in resistance of ink flow. As a result, the amount of ink ejected varies among the nozzles 3001, thereby reducing the grade of print images.
Further, the formation of the nozzles 3001 is roughly classified into a method of sticking and joining a film made of polyimide or the like and having nozzle openings already formed by a laser machining process, to the substrate 3005, or a method of coating the substrate 3005 with resin material or the like and then forming nozzle openings by using a photolithography technique to execute exposure and development or carrying out plasma etching. However, in view of the recently growing demand for prints with photographic image quality, it is expected that ink droplets will need to impact sheets more and more precisely. Accordingly, in view of the machining precision for the nozzles 3001 and the alignment precision for the heaters, it is more advantageous to collectively form the nozzles 3001 on the substrate 3005 using the photolithography technique as in the above second method.
In a side shooter type ink jet print head constructed as described above, ink is held with forming meniscus in the vicinity of the plurality of nozzles. Then, the heating resistance elements 3002 are selectively driven in accordance with image data so that the resulting thermal energy is used to rapidly heat ink on a heated surface to cause film boiling, thereby ejecting the ink using the force of the resultant bubbles.
If the resistance values of the heating resistance elements 3002 arranged on the substrate 3005, described above, vary among print heads, providing uniform drive power to all print heads varies the amount of heat generated among the heating resistance elements 3002 owing to the variation in the resistance values, thus leading to differences in ink bubbling phenomenon among the print heads. For example, if the drive power is set at a small value relative to a required resistance value, the ink is unstably ejected, i.e., the amount of ink ejected is not uniform. In contrast, if the drive power is set at a large value, an unnecessarily large amount of power is supplied to the heating resistance elements to reduce the lives of these elements or the print head, thereby possibly degrading the reliability of the print head. Accordingly, it is very desirable to solve the above problems by measuring the resistance values of the heating resistance elements 3002 of each head using a certain method to provide appropriate power for these resistance values.
However, if an attempt is made to directly measure the heating resistance elements 3002 of each head, the total resistance value, i.e. the resistance values of each heating resistance element 3002 and functional elements electrically connected thereto, is measured, thereby hindering the resistance value of only the heating resistance element 3002 to be accurately measured.
Thus, as disclosed in the applicant's Japanese Patent Application Laid-open No. 7-76077 (1995), the following methods have been employed; a method using a head construction having measuring resistance elements which are electrically independent of heating resistance elements and functional elements and which have larger resistance values than the heating resistance elements, the method comprising the steps of measuring the resistance values of the measuring resistance elements, formed similarly to the heating resistance elements and determining a sheet resistance from the resistance values of the measuring resistance elements to estimate the resistance values of the heating resistance elements, and a method of actually performing a printing operation to measure ejection threshold values. The ink can be stably ejected by setting appropriate drive power for the resistance values of the heating resistors of each head.
If one of these methods is used, i.e. the measuring resistance elements are used, it is most common to classify the heads into a number of ranks on the basis of the estimated resistance values so as to avoid problems due to differences in ink bubbling phenomenon as described above, and to set appropriate drive power, for example, an appropriate drive signal pulse width, for each rank.
As an example of such ranking, FIG. 2 shows data on a head in which power transistors for driving the heating resistance elements are of an n-MOS type and the heaters are composed of TaN. The other conditions include a heater size of 24×24 μm, a sheet resistance of 50 Ω/□±10%, a wiring resistance of 8 Ω, a power transistor ON resistance of 17 Ω±40%, a drive power of 11 V, and a correction value (K value, described later) of 1.20 for a predetermined margin. As shown in FIG. 2, the pulse width set depending on the varying resistance value of the head is between 0.82 and 1.29 μsec, and the difference between the maximum and minimum values of the pulse width is 0.47 μsec.
Further, the pulse width set for each rank is the appropriate value set per nozzle at room temperature, so that the pulse width is modulated in order to negate the adverse effects of the temperature of the head as being driven, the density of print patterns (the number of nozzles through which ink is simultaneously ejected), or the like (this modulation will hereinafter be referred to as “PWM control” or “K value control”). Specifically, control is provided such that the pulse width is reduced if the temperature increases during a head driving operation, and is increased if the density of print patterns increases.
In the recent years, ink jet printing apparatuses have advanced rapidly in the market, thereby requiring the definition of print images to be further improved. Thus, it is desirable to increase the resistances of the heating resistance elements in order to allow smaller ink droplets to be efficiently ejected. Presently, heating resistors for thermal ink jet are composed of HfB, TaN, TaAl, TaSiN, or the like, and no other high-resistance material has been discovered yet.
It is thus contemplated that the resistances of the heating resistance elements may be increased by reducing the thickness of each heater or improving its shape to substantially increase the number of sheets. However, in this case, manufacturing constraints become more severe to increase the variation in resistance value. Thus, with the above-described conventional method, print heads with a rank “min.” having the minimum resistance value within a tolerance range have an excessively small pulse width, whereas print heads with a rank “max.” having the maximum resistance value within the tolerance range have an excessively large pulse width. Further, even if the pulse width modulation control is designed so as to properly correspond to each rank at the room temperature (for example, between 15 and 35° C.), it may be very difficult to design the PWM control or K value control in accordance with a variation in temperature or print pattern density. To avoid this problem, it is contemplated that the tolerances may be reduced so that heads, the resistance value of which deviate from the tolerance, are considered to be defective. However, this may reduce the yield of manufactured print heads to sharply increase the costs thereof, so that this method is not a realistic solution.